In most prior art printers, particularly impact printers, the printhead carriage rides along a pair of parallel carriage guides as it travels back and forth through the printing zone while performing the printing function. The carriage guides usually are rigid rods which are anchored at their ends to the printer frame. These types of carriage guides typically pass through bearing surfaces disposed in cooperatively formed apertures in the printhead carriage. An example of such a device is found in U.S. Pat. No. 4,808,019. Such structures are also associated with ink jet printers, as shown in U.S. Pat. No. 4,581,618.
In other types of printing mechanisms, particularly non impact printers such as ink-jet printers, only one carriage guide passes through cooperatively formed apertures in the printhead carriage, and a separate surface of the printhead carriage rests in sliding contact with a surface on another carriage guide. The weight of the printhead carriage preloads the carriage against the other carriage guide. This insures constant contact and proper referencing of the printhead to the printer frame, to which these carriage guides again are securely attached. Although pivoting of the printhead carriage about the first carriage guide may be permitted, it rarely occurs during the printing operation. An example of such a structure is found in U.S. Pat. No. 4,872,026.
In both types of prior art printing mechanisms described hereinabove, a set of rollers is required to retrieve paper from a supply and feed the paper to the printing zone. Typically, this function is performed by one or more pinch rollers in conjunction with a drive roller. These pinch rollers are separate from the carriage guides described above. Thus, two separate mechanisms are required to perform the functions of guiding the printhead carriage and feeding the paper to the printing zone. The result is a relatively large and complex mechanism.
Furthermore, in these prior art printing mechanisms described hereinabove, both the carriage guides and the platen are fixed to the printer frame, so that the positions of the printhead and the side of the paper opposite of the printable side are also fixed with respect to the frame. A drawback of these printing mechanisms is that while the printhead is referenced to the platen, it is not necessarily referenced to the printable surface of the paper, or other printing medium. Therefore, it is difficult to print on papers of different thicknesses or weights, such as envelopes, because it is not possible to easily control the spacing between the printhead and the paper as the paper thickness changes. The user can only change the printhead to-platen spacing by making time consuming manual adjustments. Thus, with prior art machines, the user is faced with the option of printing only on paper of a given weight, or of suffering a reduction of the printing quality when different weight papers are used.
Moreover, in such prior art printing mechanisms, there is a large number of parts which, when assembled, determine the spacing between the printhead and the platen. Because each part has a manufacturing tolerance associated with it, this relatively large number of parts makes it difficult to control the printhead-to-platen spacing. These problems are particularly acute for ink jet printers in which it is important that the printhead be referenced accurately to the paper.
In the somewhat complex prior art printers described hereinabove, there are several opportunities for the paper to become jammed during the printing operation, and, because of the complexity of the printer, it is frequently difficult to clear such a paper jam readily.
Accordingly, it is an object of the present invention to provide a more simplified printing mechanism.
It is another object of the present invention to provide a printing mechanism in which the printhead is accurately and automatically referenced to the printable surface of the paper to permit accommodation of papers of different weights.
It is a further object of the present invention to provide a more compact and inexpensive printer.
It is another further object of the present invention to simplify tee paper feed in a printing mechanism.